Sensing a capacitive touch panel is achieved by cooperation between the capacitive touch panel and the hardware, firmware and software of the touch circuit. For example, a capacitive touch panel having a two-dimensional trace layout requires a two-dimensional touch circuit to sense therewith and to convert the sensed capacitance variation of each trace into a digital value, called analog-to-digital conversion (ADC) value, for position detection of the object thereon.
FIG. 1 is a schematic diagram of a typical capacitive touch panel module, which includes a component carrier 10, a capacitive touch panel 12 and a capacitive touch sensor 14 printed on the capacitive touch panel 12. The component carrier 10 is typically a flexible printed circuit (FPC) board, with a chip of integrated circuit (IC) including a detector circuit deposited thereon and connected to the capacitive touch sensor 14 through the metal wires printed on the component carrier 10.
In terms of object detection, a two-dimensional capacitive touch panel requires much less computation than an all-point-array capacitive touch panel. However, for multi-finger applications, a two-dimensional capacitive touch panel has its congenital defect, the ghost phenomenon, that is not found in an all-point-array capacitive touch panel. FIG. 2 is a schematic diagram showing the ghost phenomenon, and as it is shown, a two-dimensional capacitive touch panel 12 has X traces X1-Xm and Y traces Y1-Yn, and the conventional two-dimensional object detection includes sequential scan of all the traces X1-Xm and Y1-Yn one by one to extract the X and Y ADC values, and combination of the X and Y ADC values to locate the object position. In single-finger applications, for example, only one finger touching at the position 20, the X and Y traces are sequentially scanned for their ADC values which indicate the capacitance variations thereof, and it will find that the traces where the position 20 is have significant capacitance variations, i.e., the X trace having the largest capacitance variation is the trace X3 and the Y trace having the largest capacitance variation is the trace Y3. Thus, it is easy to identify the finger position (X3,Y3), called “real point”. However, if there are two fingers touching at the positions 20 and 22 respectively, then it will find two peaks on the X traces X3 and X10 and two peaks on the Y traces Y3 and Y10 after scanning all the X and Y traces. Therefore, from the combination of the peak ADC values, it will obtain four positions (X3, Y3), (X10, Y7), (X3, Y7) and (X10, Y3), indicated by the numerals 20, 22, 24 and 26, of which, however, the positions 24 and 26 have no fingers thereon and are called “ghost points”. Obviously, the ghost points will lead to incorrect location of the fingers.
With the gradual popularization of multi-finger applications in capacitive touch panels, the modern capacitive touch panels are required to satisfy the needs of two or more finger detection. For the ghost issue on the two-dimensional object detection, a multi-touch sensing method is proposed to obtain all-point-array data from a two-dimensional structure, which may distinguish between real points and ghost points based on the characteristic that the trace of a real point will have a significantly greater or smaller self capacitance to ground than the trace of a ghost point. FIG. 3 is a schematic diagram showing a conventional two-step sensing method and FIG. 4 is a flowchart thereof. This method still includes the step S30 to sequentially scan the X and Y traces to get all the X and Y ADC values. However, the next step S32 will identify if there is a multi-finger touch, and if it does not find any multi-finger touch, then step S34 will be performed for position calculation with the X and Y ADC values whenever the capacitive touch panel is touched. If the step 32 does find a multi-finger touch, then step S36 will be performed to carry out a multi-finger scan which includes an inphase crisscross sensing process applied to the four positions 20, 22, 24 and 26, as shown in FIG. 3 for example. In further detail, the trace Y3 is stimulated by a current signal when sensing the trace X3 to detect the self capacitance variation of the position 20; the trace Y7 is stimulated by a current signal when sensing the trace X3 to detect the self capacitance variation of the position 24; the trace Y3 is stimulated by a current signal when sensing the trace X10 to detect the self capacitance variation of the position 26; and the trace Y7 is stimulated by a current signal when sensing the trace X10 to detect the self capacitance variation of the position 22. Then, step S38 is performed to get the ADC values from the real and ghost points 20, 22, 24 and 26, and step S40 compares the ADC values to distinguish between the real points 20, 22 and the ghost points 24, 26.
As illustrated in the above description, the conventional two-step sensing method first identifies a multi-finger touch and then senses the self capacitance variation of each possible position again with particular sensing process in the latter multi-finger scan. Unfortunately, the intersection points on a capacitive touch panel may be different in self capacitance. Thus, before the sensing process of FIG. 4, it is necessary to perform the inphase crisscross sensing process for each intersection point without being touched to get the ADC values thereof, in order to determine the calibration parameter of each intersection point for its analog-to-digital conversion. The calibration parameters are to make the ADC values of all the intersection points without being touched fall in a same level range, and are stored in advance for application to the sensing of the capacitive touch panel to calibrate the sensed data each time in the future. Therefore, when the capacitive touch panel is in practice, the ADC value detected from a trace will indicate the self capacitance variation of the trace, and can be used to distinguish between real points and ghost points correctly. For a two-dimensional capacitive touch panel, if the number of the X traces is m and the number of the Y traces is n, then the conventional two-step sensing method needs to store m×n calibration parameters in advance, which requires relatively large memory.